<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script>
        document.addEventListener('DOMContentLoaded', function() {
            const btn  = document.querySelector('button');
            btn.addEventListener('click', function() {
                alert(11);
            });
            });
    
        </script>
</head>
<body>
<button>点我</button>

</body>
</html>